Fill cylinder cap with quads
If you know that your circle has a even number of vertices, and do not care how the gird is aligned; then select all the vertices in the circle with alt RMB , then ctrl F > grid fill.



Select half of the vertices of the circle. The selection needs to have the same number of vertices along the top and bottom, and the same number unselected on the side.


Then press ctrl F > grid fill.


Which will give you this.